On steering: (particularly if your box has 181k miles), you can try tightening the steering box adjustment, though at a certain point of wear in the box internals, that doesn't do much. Best case Ontario, you can remove play that way. Nothing fixes it like a new(er) box. Also consider replacing all of the steering links (apologies if you already did that, just reminding you), that removed some play in my case.
You've got most of the suspension covered, IMHO. I bought my '91 with a 100% shot suspension, so I've been through this. I did all of the steering links and arms, swaybar bushes and links, the E31 lowers, new Lemforder uppers (bought sans bushing, make sure you get those or you'll be pressing out a new bushing), and Mooseheads up front. A big difference for me in sound (in terms of clanks) and tightness was getting all 97 of those worn ball joints up front (ok, not really, but there are a LOT) replaced with fresh ball joints. I'm referring to the assort joints in all of those arms mentioned.
The rear is quite simple, I like the poly subframe bushings you already have; new dog bones, and swaybar bushes and links. As I mentioned in an earlier post, I'll bet your RTA bearings are still fine. Maybe add differential bushings to the rear and you're basically solid.
I could go on a diatribe about Bilstein being a poor match for this car, especially compared to Konis, but they'll do.
For "upgrades", you've basically just got bigger sways, and coil-overs. Since you have an M5, it's really only worth it to go aftermarket on swaybars as you have large factory bars already, and some would doubt the effectiveness of adding, say, the Nurburgring package Touring rear bar. Coil overs are a whole 'nother bag.
A cool addition, especially combined with your RD strut brace, is the E31 underbody x-brace. My touring has this (but not strut brace), but it came installed and I can't comment on what it actually changes--but it HAS to increase rigidity, one would think.